In order to extract hydrocarbons from underwater it is often necessary to provide flowlines extending from the seabed to the surface. If the flowline merely depended from a surface mounting the weight of the dependent flowline would, unless the flowline was made uneconomically strong, be likely to part. In order to prevent this buoyancy is mounted around the flowline along its length. It is not however possibly to merely bolt the buoyancy to the flowline.
All flowlines are made to a manufacturing tolerance on diameter which means that the actual diameter of the flowline will probably differ from a nominal size. Buoyancy or clamps machined to fit the nominal size flowline will be too small for a flowline larger than nominal and too small for a flowline larger than nominal. In either case the buoyancy will not be secured to the flowline with optimal efficiency and it may not be able to resist the upthrust up on it, causing it to shift.
In some cases securing mismatched sized buoyancy to a flowline may cause damage to the flowline.
Not all flowlines are of constant diameter in any event. Some flowlines, such as flexible risers, are made of material which expands or contracts in response to changes in internal pressure of the contents, the load, ambient temperature or the temperature of the flowline contents.
Additionally some flowlines are not in a straight line and bending of the flowline requires the buoyancy to be of larger diameter than when the flowline is linear.
Furthermore some flowline materials creep. Creep is long term dimensional change in response to load.
To overcome these problems the present applicants devised a so-called riser clamp which is described in GB 2 286 649. This product, since much imitated, comprises a flexible clamp which is clamped around the riser or flowline. Because the clamp is flexible it can conform closely to the flow line. A buoyancy member can be provided with a pocket which has surfaces engaging the clamp. Buoyancy load is therefore transferred from the buoyancy to the flowline.
While this invention went a considerable way towards overcoming the problem of installing buoyancy onto flowlines the solution was not perfect and a number of problems remained. Installation is time consuming and involves a large number of loose parts. When one is attempting to install the riser clamps on an exposed deck of a pipelay vessel in extreme conditions, the desirability of quick installation with few parts to get lost quickly becomes apparent.
In installing the bolts fastening the clamp together there is a danger of cross-threading the bolts with the risk of heat build-up and degradation of physical properties.
While the clamp described in GB 2 286 649 goes some way towards providing an even pressure distribution about the flowline and succeeds to a much greater extent than rigid clamps as described for example in GB 2 288 205 still more even pressure distribution would be desirable.
The invention seeks to reduce at least some of the problems with the prior art devices.